A Ni-based alloy is excellent in mechanical properties and thus used for various kinds of members. In particular, a Ni-based alloy, which is excellent in corrosion resistance, is used for members of a nuclear reactor because the members are exposed to high-temperature water. For example, 60% Ni-30% Cr-10% Fe alloy or the like is used for members of a steam generator of a pressurized water reactor (PWR).
These members are to be used for several or several tens of years under an environment of high-temperature water at around 300° C., a reactor water environment of a nuclear reactor. A Ni-based alloy used for a heat-transfer tube for steam generators for nuclear plant contains a large amount of Ni, and thus is excellent in corrosion resistance and low in corrosion rate. However, a long period of use of the Ni-based alloy causes a very small amount of Ni to be released from its base metal.
The released Ni is transported to a reactor core portion in the process of circulating reactor water and irradiated with neutrons in the vicinity of fuel. Ni irradiated with neutrons is converted into radiocobalt through nuclear reaction. This radiocobalt has a very long half-life and continuously emits radiation for a long period. Therefore, when the release amount of Ni is increased, a periodical inspection cannot be started until the amount of emitted radiation is reduced to a proper value, which prolongs a period between periodical inspections, causing economic losses.
The reduction in the release amount of Ni is very important matter from the viewpoint of using a light-water reactor for a long period. For this reason, countermeasures to prevent the release of Ni in a Ni-based alloy have been taken so far by improving the corrosion resistance of a material and controlling the water quality of reactor water.
Patent Document 1 discloses a method in which a heat-transfer tube of a Ni-based alloy is subjected to annealing in an atmosphere at a degree of vacuum of 10−2 to 10−4 Torr and within a temperature range of 400 to 750° C. to form an oxide film mainly containing a chromium oxide, improving general corrosion resistance.
Patent Document 2 discloses a method for manufacturing a member for nuclear power plant in which heat treatment is performed in an oxidizing atmosphere at 10−3 Torr to atmospheric pressure after the solution treatment of a Ni-based precipitation strengthened alloy, the heat treatment also serving as at least part of age hardening treatment and oxide film forming treatment.
Patent Document 3 discloses a method for manufacturing a Ni-based alloy product in which a Ni-based alloy product is subjected to heat treatment in a hydrogen atmosphere or a mixed atmosphere made of hydrogen and argon, both the atmospheres having a dew point ranging from −60° C. to +20° C.
Patent Document 4 discloses a method in which an alloy work piece containing nickel and chromium is exposed to a gas mixture made of steam and at least one kind of nonoxidative gases to form a chromium enriched layer.
Patent Document 5 discloses, as a heat treatment method by which a double-layered oxide film is generated on the inner surface of a Ni-based alloy tube reliably and with a high degree of efficiency, the double-layered oxide film suppressing the release of Ni under a high-temperature aqueous environment, a heat treatment method in which at least two gas supply devices are provided on the delivery side of a continuous heat treat furnace, or a gas supply device is provided on each of the delivery side and the entrance side of the continuous heat treat furnace, then, using a gas introduction tube that penetrates the furnace and one of these gas supply devices, the inside of a working tube to be charged into the heat treatment furnace is supplied with an atmospheric gas made of hydrogen or mixed gas of hydrogen and argon, the atmospheric gas having a dew point ranging from −60° C. to +20° C., from a distal end side in the travelling direction of the working tube, and the working tube is charged into the furnace and retained at 650 to 1200° C. for 1 to 1200 minutes. At this point, the operation of switching the supply of the atmospheric gas to the inside of the working tube to the supply from another gas supply device after the distal end of the working tube reaches the delivery side of the furnace, is repeated.
Patent Document 6 discloses a method for manufacturing in which a Cr-containing nickel-based alloy tube is treated in an atmosphere made of a nonoxidative gas containing carbon dioxide to form a chromium oxide film having a predetermined thickness on the inner surface of the tube.